1 . Field of the Invention
The present invention relates generally to automatic transmissions, and more particularly to a transmission control system with extreme cold logic.
2 . Background and Summary of the Invention
Conventional automatic transmissions include a hydrodynamic torque converter to transfer engine torque from the engine crankshaft to a rotatable input member of the transmission through fluid flow forces. The transmission includes frictional units which couple the rotating input member to one or more members of a planetary gear set. Other frictional units hold members of the planetary gear set stationary during flow of power. The drive clutch assemblies can couple the rotating input member of the transmission to the desired elements of the planetary gear sets, while the brakes hold elements of these gear sets stationary. Such transmission systems also typically provide for one or more planetary gear sets in order to provide various ratios of torque and to ensure that the available torque and the respective reactive power demand are matched to each other.
Automatic transmissions are typically provided with a hydraulic control system and associated electronic controller which take automatic control of the friction units, gear ratio selecting, and gear shifting. The hydraulic control system includes various fluid passages connected between valves which are controllable for applying and releasing the clutch elements in order to shift the gears of the transmission based upon the controls provided by the electronic controller. The electronic controller typically chooses the proper gear dependant upon a shift program selection, accelerator position, engine condition, and vehicle speed. The hydraulic control system uses several valves to direct and regulate the supply of hydraulic pressure to the clutch elements. The hydraulic pressure control causes either the actuation or de-actuation of the respective frictional clutch units for effecting gear changes in the transmission. The valves used in the hydraulic control circuit typically comprise spring-biased spool valves, spring-biased accumulators and ball check valves. Since many of the valves rely upon springs to provide a predetermined amount of force, it will be appreciated that each transmission design represents a finely tuned arrangement of interdependent valve components.
With the automatic transmission assembly relying upon hydraulic fluid for actuation or deactuation of the respective frictional clutch units for effecting gear changes in the transmission, the viscosity of the hydraulic fluid is an important factor in providing sufficient control of the frictional clutch units. In particular, under extreme cold temperatures, the hydraulic fluid can take on a viscosity similar to molasses. Specifically, below approximately -16.degree. F., the viscosity of the hydraulic fluid prevents adequate real-time closed loop control of pressure to the clutch elements with electronic controls. Accordingly, it is an object of the present invention to provide an automatic transmission with a control system which includes an extreme cold logical shift strategy which provides a relatively high gear ratio launch and a limited shift to a lower gear ratio which requires the application of a single clutch member with no risk of torque overlap between elements.
According to this aspect of the present invention, a hydraulic control system is provided for an automatic transmission including a planetary gear system having a clutch element engagable to provide a first relatively high torque ratio and a second clutch element engagable along with the first clutch element in order to provide a second relatively lower gear ratio. The control system includes an extreme cold shift logic strategy which senses when the transmission fluid is below a predetermined temperature and institutes an extreme cold shift logic wherein transmission shifting in forward gears is limited to a first and a second gear ratio.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.